Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 690

the UK claim to have produced an electrolyser that can operate with renewable
sources, at a cost of $164/kW, and are currently planning to begin mass production
in 2008 [26]. Maintenance costs are expected to be 3% of the capital cost [2].

4.9.1.2 Future of hydrogen production
Immediate developments are investigating the possibility of producing an elec-
trolyser that can pressurise the hydrogen during electrolysis, as compressing the
hydrogen after production is expensive and unreliable. Like all areas of HESS, the
electrolyser needs a lot more development as well as technical maturity.

4.9.2 Hydrogen storage
A number of different options are currently available to store hydrogen:

1. Compression : The hydrogen can be compressed into containers or underground
reservoirs. The cost of storing hydrogen in pressure vessels is $11/kWh to
$15/kWh [2]. However, for underground reservoirs it is only $2/kWh [27].
This is a relatively simple technology, but the energy density and effi ciency
(65-70%) are low. Also, problems have occurred with the mechanical compres-
sion. However, this is at present the most common form of hydrogen storage
for the transport industry, with the hydrogen compressed to approximately 700
bar (the higher the storage pressure, the higher the energy density, see Fig. 16).
Although the energy required for the compression is a major drawback.

2. Liquefi ed hydrogen : The hydrogen can be liquefi ed by pressurising and cooling.
Although the energy density is improved, it is still four times less than conven-
tional petrol. Also, keeping the hydrogen liquefi ed is very energy intensive, as
it must be kept below 20.27 K [28].

3. Metal hydrides : Certain materials such as nanostructured carbons and clathrate
hydrate absorb molecular hydrogen. By absorbing the hydrogen in these ma-
terials, it can be easily transported and stored. Once required, the hydrogen is
removed from the parent material. The energy density is similar to that obtained
for liquefi ed hydrogen [28]. The extra material required to store the hydrogen is
a major problem with this technique as it creates extra costs and mass. This is
still a relatively new technology, so with extra development it could be a viable
option; especially if the mass of material is reduced. Carbon-based absorption
can achieve higher energy densities, but it has higher costs and even less dem-
onstrations [2]. Both metal hydride or carbon-based absorption use thermal en-
ergy. This thermal heat could come from the waste heat of other processes with
HESS, such as the electrolyser or fuel cell (FC), to improve overall effi ciency.

Each storage technique is in the early stages of development and hence there is no
optimum method at present with research being carried out in each area.

4.9.3 Hydrogen usage
There are two superior ways of using hydrogen:

1. internal combustion engine (ICE)

2. fuel cell